This invention relates to focusing arrangements for electron beam emitting cathodes and more particularly to apparatus for focusing such electron beams onto an anode in x-ray tubes.
As is known in the art, in typical x-ray tubes x-rays are generated by bombarding a target area, typically comprising a tungsten-rhenium (W-Re) alloy, of an anode made from suitable metal, such as titanium-zirconium-molybdenum (TZM), with an electron beam generated by a cathode. Typically, the cathode comprises a helical-shaped filament, transversely disposed with respect to the anode, with the length dimension of the filament positioned substantially parallel to the anode target area. The filamentary cathode is heated to electron-emissive temperatures by passing an alternating current (i.e., AC) electrical signal therethrough, such AC signal typically riding on a high negative DC potential with respect to ground. The anode, which may be a rotating anode, is commonly held at a high positive DC potential with respect to ground to thereby create a large potential difference (for example, approximately 120 KVDC) between the cathode filament and the anode. The helical filament is conventionally supported within an opening in an electrically conductive body portion of the cathode. The body portion is held at a predetermined electrical potential with respect to ground, typically at approximately the same negative DC potential as that applied to the filament, to electrostatically focus the beam of electrons emitted by the cathode filament onto a predetermined region, commonly referred to as the electron beam focal spot, of the anode target area, from which x-rays are emitted. The focused electron beam has a generally rectangularly-shaped electron distribution at the electron beam focal spot, the length of which corresponds to the filament length and the width of which comprises a central portion, referred to as the A-distribution, and a pair of peripheral portions, known collectively as the B-distribution, located on either side of the central portion.
Often, it is desirable to limit the electron beam focal spot to the smallest possible size, since this results in the highest resolution x-ray imaging. Prior art systems have variously altered the geometry of the cathode body opening, the location of the filament within the opening, the spacing between the filamentary cathode and the anode, and the biasing potential on the cathode body in an attempt to coalesce the A-distribution and B-distribution at the electron beam focal spot to thereby reduce electron beam focal spot size. One such prior art system includes an opening comprising a small slot disposed in a larger slot within the cathode body, with the helical filament being disposed near the junction of the two slots. While such a system has operated satisfactorily in some applications, other applications require electron beam focal spot area smaller than the electron beam focal spot area that such prior art system provides
Another system, described in U.S. Pat. No. 3,646,379, entitled, "X-Ray Tube Having Controllable Focal Spot Size" and assigned to the present assignee, further disposes a pair of metal tabs, biased at zero or negative potential with respect to the filamentary cathode, at selected positions between the filament and the anode in the path of the electron beam emitted by the filament. While such system functions satisfactorily in some applications, the apparatus reduces the length of the electron beam focal spot while not significantly reducing the width thereof. That is, such apparatus does not substantially reduce the B-distribution from the electron beam focal spot and, moreover, reduces the electron beam current by blocking significant portions of the filamentary cathode along the length thereof from the anode.